Biological treatment of wastewater for removal of dissolved organics is well known and is widely practiced in both municipal and industrial plants. This biological process is generally known as the “activated sludge” process in which micro-organisms consume organic compounds through their growth. The process necessarily includes sedimentation of the microorganisms or “biomass” to separate it from the water and complete the process of reducing Biological Oxygen Demand (BOD) and Total Suspension Solids (TSS) in the final effluent. The sedimentation step is typically done in a clarifier unit. Thus, the biological process is constrained by the need to produce biomass that has good settling properties. These conditions are especially difficult to maintain during intermittent periods of high organic loading and the appearance of contaminants that are toxic to the biomass.
Typically, an activated sludge treatment has a conversion ratio of organic materials to sludge of up to about 0.5 kg sludge/kg COD (chemical oxygen demand), thereby resulting in the generation of a considerable amount of excess sludge that must be disposed of The expense for the excess sludge treatment has been estimated at 40 to 60 percent of the total expense of a wastewater treatment plant. Moreover, a conventional disposal method of landfilling sludge may cause secondary pollution problems. Therefore, interest in methods to reduce the volume and mass of excess sludge has been growing rapidly.
Membranes coupled with biological reactors for the treatment of wastewater are well known but are not widely used. In these systems, ultrafiltration (UF), microfiltration (MF), or nanofiltration (NF) membranes replace sedimentation of biomass for solids-liquid separation. A membrane can be installed in a bioreactor tank or in an adjacent tank where mixed liquor, continuously pumped from the bioreactor tank and back, produces effluent with much lower total suspended solids (TSS), typically less than 5 mg /L, compared to 20 to 50 mg/L from a clarifier.
More importantly, membrane biological reactors (MBR) de-couple the biological process from the need to settle the biomass, since the membrane sieves the biomass from the water. This allows operation of the biological process at conditions that would not be desirable in a conventional system including: (1) high mixed liquor suspended solids (bacteria loading) of 10 to 30 g/L; (2) extended sludge retention time; and (3) short hydraulic retention time. In a conventional system, such conditions may lead to sludge bulking and poor settleability.
The benefits of an MBR operation include low sludge production, complete solids removal from the effluent, effluent disinfection, combined COD, solids and nutrient removal in a single unit, high loading rate capability, and minimal problems with sludge bulking Disadvantages include aeration limitations, membrane fouling, and membrane costs.
Membrane fouling can be attributed to surface deposition of suspended or dissolved substances. An MBR membrane interfaces with the biomass which contains aggregates of bacteria or “flocs”, free bacteria, protozoan, and various dissolved microbial products (SMP). The term SMP has been adopted to define the organic compounds that are related into the bulk microbial mixed liquor from substrate metabolism (usually biomass growth) and biomass decay.
In operation, the colloidal solids and SMP have the potential of depositing on the surface of the membrane. Colloidal particles form layers on the surface of the membrane, called a “cake layer”. MBR processes are designed to use rising coarse air bubbles to provide a turbulent cross flow velocity over the surface of the membrane. This process helps to maintain the flux through the membrane, by reducing the buildup of a cake layer at the membrane surface.
Compared to a conventional activated sludge process, floc (particle) size is reportedly much smaller in typical MBR units. Small particles can plug the membrane pores, a fouling condition that may not be reversible. Since MBR membrane pore size varies from about 0.04 to about 0.4 micrometers, particles smaller than this can cause pore plugging. Pore plugging increases membrane resistance and decreases membrane flux.
Efficient and stable operation of MBR systems largely depends on the conditions and qualities of the biological populations of the biomass in the MBR system. The characteristics of the mixed liquor, including viscosity, extracellular polymeric substances (EPS), floc size, and colloidal and soluble organic substances, affect membrane filterability. While traditional approaches mostly rely on optimization of hydrodynamics and air scouring to reduce membrane fouling in MBR systems, new efforts are more devoted to coagulate and flocculate the activated sludge by adding chemicals and thereby to bind colloids and other mixed liquor components in flocs. These filterability enhancement chemicals not only have a positive impact to decrease soluble foulants in the bulk phase, but also improve the hydraulic permeability of the cake formed on the surface of the membrane.
Recently, increasing efforts have been devoted to improving microbial mixed liquor filterability and enhance membrane flux in MBR systems. Options include use of inorganic coagulants such as ferric and aluminum salts and aluminum polymers, powdered activated carbon (PAC) and other types of inert particles (e.g., resins), and water soluble polymers. Use of inorganic coagulants will increase sludge generation and are only applicable to a narrow pH range. Addition of powdered activated carbon to MBR systems will not only increase sludge concentration, it may also cause irreversible permeability loss due to membrane pore plugging by PAC, and membrane wear due to the abrasiveness of the PAC. These problems will exaggerate, and additional fouling may develop when the added PAC concentration becomes higher (e.g., 600 mg/L or above).
Accordingly, there is a need for effective treatment for membrane flux enhancement, MBR efficiency improvement, and mixed liquor filterability enhancement.